Zeolites are crystalline aluminosilicates constructed by AlO4 or SiO4 tetrahedra with various framework structures that are extensively applied in adsorption, catalysis and separation. Generally, conventional zeolites are composed of 8, 10, or 12-membered-ring (12-MR) pore channels with pore sizes less than 1 nm. Due to their excellent stability, strong acidity and regular pore sizes, these microporous crystalline materials are of great importance to industrial catalysis as heterogeneous catalysts in petrochemical and chemical conversion processes. However, when large species with sizes similar with or greater than the dimensions of the pores in zeolites are involved in a catalytic conversion, the active sites in zeolites tend to become inaccessible due to strong diffusion limitation or molecular rejection induced by the relatively rigid zeolite micropore structure, which results in a less effective use of zeolite catalysts. Moreover, the diffusion limitation of reaction products or intermediates also increases the possibility of coking, or changes in desired product distribution.
To alleviate these challenges, the concept of hierarchical zeolites were suggested to introduce at least one additional pore system, usually in the mesopore range (i.e. mesoporous zeolites), in conventional zeolites. Zeolite beta has a three-dimensional network of 12-MR pores featuring an intergrowth of two or more polymorphs with pore diameters of 0.55×0.55 nm and 0.76×0.64 nm and is one of the several most important zeolites in the refining and fine chemical industries. Various strategies have been reported for the synthesis of hierarchical beta zeolites. Hierarchical beta structures with tunable mesoporosity have been synthesized with various mesopore templates, such as cationic polymers (i.e., polydiallyldimethylammonium chloride) and ammonium modified chitosan or structured organosilanes with different functional groups.
The presence of mesopore templates usually enhance the interaction within silicate species and avoid phase separation during the process of “mesopore template-zeolite structure directing agent” dual-templating synthesis. Alternatively, hierarchical beta structure with intercrystalline mesopores is assembled without using mesopore templates by nanocrystal aggregation in a dry-gel conversion process.
Hierarchical MFI and Y zeolite structures have been obtained by direct base leaching of microporous MFI and Y zeolites. This method is usually referred as a top-down approach or desilication. In the top-down approach, mesoporosity is created by partial dissolution and recrystallization of the zeolite framework in an alkaline solution. However, the framework of zeolite beta is less stable than MFI zeolites, and amorphization easily occurs for high Si beta zeolites during NaOH leaching, which may negatively impacts the microporosity and acidity of the resulting products. Moreover, it is not clear how to manipulate the mesopore sizes of mesoporous beta zeolites by the top-down approach.